The present invention relates to a method of producing polyethylene terephthalate from an aromatic dicarboxylic acid or the derivative thereof and a glycol in the case of producing a synthetic high molecular resin, e.g. a production step of polyester. More particularly, the invention relates to a process of controlling a reaction such that an analyzed value during substantially continuously analyzing a reaction liquid becomes a desired value for stably producing a low polymer having desired properties in a synthesis step, e.g. in an esterification step.
As one of the industrial production processes of synthetic resins, such as, for example, polyethylene terephthalate useful for fibers, films, or molding materials a direct polymerization process of continuously producing polyethylene terephthalate by obtaining bis-.beta.-hydroxyethyl terephthalate and/or a low polymer by the esterification reaction in the synthesis step of terephthalic acid and ethylene glycol and then by polycondensing the product is well known. In a continuous polymerization apparatus being used for such a continuous polymerization process, the molten viscosity (hereinafter, is referred to as simply "viscosity") of the polymer formed is measured at the end of the polymerization reaction and the polymerization degree of the polymer is controlled, for example, by changing the vacuum of the polymerization reactor such that the viscosity becomes an desired value, as described in JP-A-52-5895 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
The viscosity of the polymer synthesized is the index of the properties of the polymer, such as the molecular weight, etc., and is very important in the case of applying a necessary work such as an extrusion, an elongation, etc., to the polymer obtained. For example, if the viscosity is increased, an excessive load is applied to an extruding machine, whereby the polymer can not stably extruded. Also, at the elongation of the polymer, the tension is too increased to be elongated to a desired width and also large troubles such as the breakage of the film of the polymer occur. On the contrary, if the viscosity is lowered, serious problems for product quality that the film of the polymer is slackened at the elongation to reduce the flatness of the film, etc., occur. Accordingly, it is usually necessary that the viscosity of the polymer is restrained in the fluctuation range of about 50 poises to the desired viscosity.
However, in the case of changing the vacuity of the polymerization reactor for adjusting the viscosity of the polymer formed to an desired value, since it takes several tens minutes to several hours to transfer the polymer to a viscosity measuring apparatus from the polymerization reactor in the conventional process, the viscosity is reluctant to be settled even by carrying out self controlling by making a feedback loop, thereby it is inevitable to make dull the response to the change of the viscosity.
Accordingly, for adjusting the viscosity of the polymer to the desired value, it is necessary to restrain the change of the viscosity as completely as possible by maintaining other conditions as uniformly as possible, for example, the temperature and pressure of the esterification and/or the polycondensing system, etc..
However, even when the foregoing process is practiced, since the viscosity which is one of the properties of the polymer is measured at a last process of the polymerization and is not directly measured during the polymerization process, there is the problem that when the process is fluctuated by the occurrence of unexpected external disturbances such as, for example, the change of the supplying amount of ethylene glycol, the change of the supplying amounts of additives, etc., it takes several hours to several tens hours before appearing a change in the viscosity of the polymer being produced and hence the abnormality can not frequently be detected immediately, whereby the operation of the apparatus is continued for a while with the occurrence of the inferior esterification, the abnormal polymerization, etc., which results in giving the high possibility of producing a large amount of non-standardized products.
Also, when the supplying amounts of raw materials are increased for increasing the amounts of products, in particular, the esterification becomes unstable and as the case may be, the polymerizing property is greatly lowered and the polymer obtained can not sometimes form a film. Or, even when the polymer can form a film, there is a problem that the physical properties of the film, such as the flatness of the film, the tear resistance of the film, etc., are not uniform to reduce the commercial value of the product.